Electric thermal switch



June 16 1942- w. H. BULPl-r'r ELECTRIC THERMAL swITcH Filed Sept. l2,1938 Patented June 16, 1942 ELECTRIC THERMAL SWITCH Walter HenryBulpitt, Birmingham, England Application September 12, 1938, Serial No.229,625 In G-reat Britain November 18, 1937 (Cl. Mil-38) 14 Claims.

This invention relates to electric switches incorporating a safetydevice which is normally held in a set position by means of fusiblemetal, said metal melting at a predetermined temperature so as to tripsaid device, which then operates to break the circuit associated withthe switch.

The invention has particular reference to thermal switches of the typein which the safety device comprises an actuating member for the switch,a spring tending to move said member in a manner to separate the switchcontacts, a ratchet wheel normally held against rotation by said fusiblemetal, and a pawl co-operating withsaid ratchet wheel, said pawl beingconnected with said actuating member so that the latter is maintained inits retracted position until melting of the fusible metal releases saidratchet wheel.

The chief object of the present invention is to provide an improvedconstruction of thermal switch which is particularly suitable for use inconnection with immersion heaters.

Referring to the drawing:

Figure l is a side elevation of a kettle provided with an immersionheater embodying the prese ent invention.

Figures 2 and 3 are, respectively, a sectional side elevation, and aplan View of the lower part of the kettle shown in Figure 1, and aredrawn to an enlarged scale.

Figure 4 is a section on the line 4 4 of Figure 3.

In the construction of thermal switch illustrated in the drawing, theswitch proper is conplug-and-socket coupling of known form, that is tosay, it may comprise a stationary part 5 carrying contact pins 6 whichco-operate with conducting sockets 'I in the movable part 8 of thecoupling.

An appliance, such as an immersion heater 9 which requires to beprotected against over-heating, is connected across the contacts on onepart of the coupling (for example, the pins 6 on the stationary part 5),which latter may be secured in an opening I0 in the wall of a kettle IIor other vessel containing liquid to be heated. The contacts of theother coupling part 8 are connected to supply leads so that the circuitthrough the appliance is completed when the two parts 5, 3 of thecoupling are brought together.

Associated with the two-part coupling is a safety device placed in sucha position as to be subjected to a temperature which is at least that ofthe immersion heater 9 or other appliance to be protected.

In the construction illustrated, the loop-shaped body of the heater 9 isof tubular metallic construction and contains a resistance wire I2 whichmay be wound upon a mica strip I3 or other suitable support, the ends ofthe wire I2 being connected to the contact pins 6. The tube forming thebody aforesaid is seamless and has a flat section whose major dimensionis substantially perpendicular to the plane of the loop. It will be seenthat the loop is of incomplete circular shape, the terminal portions ofthe body being turned outwardly into substantially parallel relationshipand secured to the inner end of a hollow metal plug I4 which is mountedin the opening I8 aforesaid in the vessel wall.

With an immersion heater of this type, the safety device aforesaid ispreferably arranged as described in our co-pending application Serial No. 229,623 filed September '12, 1938, being mounted directly upon thebody of the heater 9 at a position on the loop substantially opposite tothe connection of the terminal portions of such body to the plug I4,which carries the stationary part 5 of the coupling.

The safety device includes a ratchet wheel I5 mounted in a housing I6which is secured by soldering or otherwise in direct metallic contact u;with the body of the heater 9. The ratchet wheel l5 may be inserted intothe housing I6 through an opening I'I in one side of the latter, andthis opening Il may be closed by a screwed plug I8 having in itsinnerend a central socket I9 adapted stituted by a two-part coupling, whichmay be a t0 receive a short co-aXial stem 20 formed on or fixedly unitedwith the wheel I5. The socket i9 contains a mass 2| of fusible metal,such as solder, which normally prevents rotation of the ratchet wheel I5in its housing I6.

The ratchet wheel housing I5 is conveniently formed as a metal plughaving a drum-shaped cavity 22 which accommodates the wheel I5 itself,and a tangential passage 23 which breaks through the wall of the cavity22 aforesaid. This passage 23 is open at its inner end, into whichprojects one extremity 24 of a push-rod 25 whose other end 26 projectsthrough a hole 35 in the stationary part 5 of the coupling. When thepart 5 is provided with two contact pins 6 as shown, the end 26 of thepushrod 25 may be arranged mid-way between these.

The end 215i of the push-rod 25 adjacent the ratchet wheel l5 has ahooked portion 21 which acts upon the teeth of the wheel I5 in themanner of a pawl.

The push-rod 25 is enclosed in a tube 28 which is suitably secured tothe ratchet wheel housing I6 and to the metal plug I4 carrying thecoupling part 5. This tube 28 may be of circular section and may have ascrewed connection with a projecting cylindrical neck 36 on the housingIt, this neck 36 being co-aXial with the tangential passage 23aforesaid. It will be appreciated that, in the case of the immersionheater shown, this tube 2S protects the push-rod 25 and associated partsfrom contact with the liquid in the vessel II. Y

The tube 28 aforesaid is of greater crosssec tion than the push-rod 25and may be contracted at a position intermediate its ends so as toprovide a bearing for the push-rod 25, which bearing may be ofsubstantial length.

A helical coil spring 29 threaded over the push rod 25 is engagedbetween a shoulder 33 on the latter and the end of the housing neck 35'or a i washer 3| abutting such neck. The effect of this spring 29 is tourge the push-red along the tube 28 towards the movable part S of thecoupling.

`The length of the push-rod 25 is such that when the spring 29 iscompressed, the end 26 of the push-rod is flush with the face 82 of thestationary coupling part 5, or only projects slightly therefrom so thatit does not prevent the two parts 5, 8, of the coupling from beingconnected together.

In this position of the push-rod 25, the hook or pawl portion 21 at itsother end engages with a tooth of the ratchet wheel I5, and as thelatter is prevented from rotating by the fusible metal 2 I, the deviceis maintained in this set position. To allow the length of the push-rod25 to be adjusted, the latter may be formed in two aligned parts whichscrew into a connecting sleeve, and the end of this sleeve may form theshoulder aforesaid against which the spring 29 acts.

In operation, when the heater 9 or other appliance associated with thesafety device reaches an excessive temperature, the fusible metal 2!melts and allows the ratchet wheel I5 to rotate due to the action of thespring 29 on the pushrod 25. This allows the hook or pawl portion 21 atthe end 24 of the push-rod 25 to slip past the tooth of the ratchetwheel I5 which it has been engaging, and the spring 29 then acts toproject the other end 26 of the push-rod 25 through the stationary part5 of the coupling, so that the movable coupling part 8 is forced out ofengagement with the contact members 1 on the stationary part 5 and thecircuit is broken. The longitudinal movement of the push-rod 25 may belimited by engagement of a second shoulder 33 thereon with a washer 34through which the end part 26 of the push-rod passes, and which abutsagainst the coupling part 5,

To re-set the device, the push-rod 25 is simply forced inwards againstthe action of the spring 28 until the hook or pawl portion 21 aforesaidengages behind the next tooth on the ratchet wheel I5. There is nonecessity, as in some types of safety device, to effect this re-settingmovement whilst the fusible metal is still fluid.

To facilitate the re-engagement of the hook or pawl 21 with the teeth ofthe ratchet wheel I5, its end face may be rounded or bevelled, and theterminal portion 24 of the push-rod 25 is allowed considerable clearancein the passage 23 in the ratchet wheel housing IG, so that it candeflect laterally before snapping into engagement with the ratchettooth. To increase its resilience, this terminal portion 24 of thepush-rod 25 may be reduced in cross-section.

It will be understood that, instead of forming the pawl 21 as part ofthe push-rod 25, it may be constructed as a separate spring detentattached thereto.

What I claim then is:

l. An electrical thermal switch comprising two coupling members one ofwhich carries ccntact pins normally in engagement with sockets on theother member, a push-rod extending through one of said coupling memberscentrally of and parallel to said contact pins and engaging the other ofsaid members, a spring urging said push-red longitudinally in adirection to separate said coupling members, a pawl connected to saidpush-rod, a ratchet wheel (zo-operating with said pawl a downwardlyprojecting cylindrical stem on said ratchet wheel, a socket in whichsaid stem is disposed, the cylindrical surface of said stem engaging thecorresponding wall of the socket and being centralised thereby, and amass of fusible metal disposed between th'e bottom of the socket and theend of the stem, said metal being adapted when solid to prevent rotationof said ratchet wheel, whereby said ratchet wheel is enabled to holdsaid push-rod in an inoperative position against the action of saidspring, said fusible metal being adapted to melt at a predeterminedtemperature, so that said coupling members are separated by theresultant movement of said push-rod.

2. An electrical thermal switch comprising a pair of coupling membersone of which carries contact pins normally in engagement with sockets inthe other member, a push-rod extending through one of said couplingmembers midway between and parallel to said contact pins and engagingthe other of said members, a spring urging said push-rod longitudinallyin a direction to separate said contact pins from said sockets, a pawlconnected to said push-rod, a ratchet wheel co-operating with said pawla downwardly projecting cylindrical stem on said ratchet wheel, a socketin which said stem is disposed, the cylindrical surface of said stemengaging the corresponding wall of the socket and being centralisedthereby, a mass of fusible metal disposed between the bottom of thesocket and the end of the stem, said metal being adapted when solid toprevent rotation of said ratchet wheel, whereby said ratchet wheel isenabled to hold said pushrod in an inoperative position against theaction of said spring, said fusible metal being adapted to melt at apredetermined temperature so that said coupling members are separated bythe resultant movement of said push-rod, and a tube en'- closing saidpush-rod and said spring.

3. An electrical thermal switch comprising two coupling members one ofwhich carries contact pins normally in engagement with sockets on theother member, a push-rod extending through one of said coupling memberscentrally of and parallel to said contact pins and engaging the other ofsaid members, a spring urging said push-rod longitudinally in adirection to separate said coupling members, a tube enclosing saidpushrod and said spring, a pawl connected to said push-rod, a ratchetwheel, a housing for said ratchet wheel, said pawl extending into saidhousing and co-operating with said ratchet wheel, a downwardlyprojecting cylindrical stem on said ratchet wheel, a socket in whichsaid stem is disposed, the cylindrical surface of said stem engaging thecorresponding wall of the socket and being centralised thereby, and amass of fusible metal disposed between the bottom of the socket and theend of the stem, said metal being disposed within said housing andadapted when solid to prevent rotation of said ratchet wheel, wherebysaid ratchet Wheel is enabled to hold said push-rod in an operativeposition against the action of said spring, said fusible metal beingadapted to melt at a predetermined temperature so that said couplingmembers are separated by ythe resultant movement of said push-rod.

4. An electrical thermal switch comprising two coupling members one ofwhich carries contact pins normally in engagement with sockets on theother member, a push-rod extending through one of said coupling memberscentrally of and parallel to said contact pins and engaging the other ofsaid members, a spring urging said push-rod longitudinally in adirection to separate said coupling members, a pawl connected to saidpush-rod, a ratchet wheel, a housing for said ratchet wheel, saidhousing having an opening through which said ratchet wheel isintroduced, said pawl extending into said housing and cooperating withsaid ratchet wheel, a co-axial cylindrical stem on said ratchet wheel, ahollow plug adapted to iill said opening and having a socket thereinwhich receives said stem and engages its cylindrical surface, and a massof fusible metal adhering within the bottom of said socket and adheringto the end of the stem and adapted when solid to prevent rotation ofsaid ratchet wheel, whereby said ratchet wheel is enabled to hold saidpush-rod in an inoperative Y position against the action of said spring,said fusible metal being adapted to melt at a predetermined temperatureso that said coupling members are separated by the resultant movement ofsaid push-rod.

5. An electrical thermal switch for controlling an immersion heater,comprising a plug having said heater mounted thereon and adapted to besecured in the wall of the liquid container associated with said heater,contact elements carried by said plug and adapted for engagement withcomplementary contact elements carried by a movable coupling member tocomplete the circuit through said heater, a push-rod extending throughsaid plug midway between and parallel to the contact elements associatedtherewith and engaging said movable coupling member, a spring urgingsaid push-rod longitudinally in a direction to separate the two sets ofcontact elements, a pawl connected to said push-rod, a ratchet wheelcri-operating with said pawl, a mounting for said ratchet wheel attacheddirectly to said heater at a position remote from said plug, and a massof fusible metal in said mounting adapted when solid to prevent rotationof said ratchet wheel, whereby the latter is enabled to hold saidpushrod in an inoperative position against the action of said spring,said fusible metal being adapted to melt at a predetermined temperatureso that the two sets of contact elements are separated by the resultantmovement of said push-rod.

6. An electric thermal switch for controlling an immersion heater of thelooped type, comprising a plug having the ends of the looped heatersecured therein and adapted to be mounted in the liquid containerassociated with said heater, contact elements carried by said plug andadapted for engagement with complementary contact elements carried by amovable coupling member to complete the circuit through said heater, apush-rod extending through said plug midway between and parallel to thecontact elements associated therewith and engaging said movable couplingmember, a spring urging said push-rod longitudinally in a direction toseparate the two sets of contact elements, a pawl connected to saidpush-rod, a ratchet wheel cooperating with said pawl, a housing for saidratchet wheel mounted exteriorly of said heater and in direct metallicconnection with the same at a position remote from the ends thereof, anda mass of fusible metal disposed within said housing and adapted whensolid to prevent rotation of said ratchet wheel whereby the latter isenabled to hold said push-rod in an inoperative position against theaction of said spring, said fusible metal being adapted to melt at apredetermined temperature so that the two sets of contact elements areseparated by the resultant movement of said push-rod.

7. An electric thermal switch for controlling an immersion heater of thelooped type, comprising a plug having the ends of the looped heatersecured therein and adapted to be mounted in the liquid containerassociated with said heater, contact elements carried by said plug andadapted for engagement with complementary contact elements carried by amovable coupling member t0 complete the circuit through said heater, apush-rod extending through said plug midway between and parallel to thecontact elements associated therewith and engaging said movable couplingmember, a spring urging said push-rod longitudinally in a direction toseparate the two sets of contact elements, a pawl connected to saidpush-rod., a ratchet wheel co-operating with said pawl, a housing forsaid ratchet wheel mounted exteriorly of said heater and in directmetallic connection with the same at a position remote from the endsthereof, a tube connected at its ends to said housing and said plug soas to enclose said push-rod and said spring in a liquid tight manner,and a mass of fusible metal disposed within said housing and adaptedwhen solid to prevent rotation of said ratchet wheel whereby the latteris enabled to hold said pushrod in an inoperative position against theaction of said spring, said fusible metal being adapted to melt at apredetermined temperature so that the two sets of contact elements areseparated by the resultant movement of said push-rod.

8. In a safety appliance for electrically heated apparatus, a metallicbase adapted to support a heating element, an insulating member carriedby said base, a plunger guidingly supported in said member forreciprocable movement and having a tooth at its innermost end, a springsurrounding said plunger and biasing its movement in an outwarddirection, a toothed ratchet wheel rotatably mounted in said base, atooth of said wheel being engageable by the tooth on said plunger whenthe plunger is depressed against its spring, fusible means normallyretaining said wheel against rotation at normal temperatures, butreleasing it for rotation at abnormal temperatures, and a circuit plugconnector arranged to depress said `plunger when moved tocircuit-connecting position, and to be moved to circuit-disconnectingposition when the plunger is released relative to said Wheel.

9. In a safety appliance for electrically heated apparatus, a metallicbase adapted to support a heating element, an insulating member carriedby said base, an elongated straight plunger guidingly supported in saidmember for reciprocable movement and having a single laterally extendingtooth at its inner most end, a spring surrounding said plunger andbiasing-its movement in an outward direction, a toothed ratchet wheelrotatably mounted in said base, a tooth of said wheel being engageableby the tooth on said plunger when the plunger is depressed against itsspring, fusible means normally retaining said wheel against rotation atnormal temperatures but releasing it for rotation at abnormaltemperatures, and an externally disposed plug connector arranged todepress said plunger when moved to circuit-connecting position,` and tobe moved to circuitdisconnecting position when the plunger is releasedrelative to said wheel.

10. An electrical thermal switch comprising a plug and socket connectorincorporating a stationary part having a pair of contact pins, thestationary part having an axial opening disposed midway between saidpins, and a movable part having a pair of sockets adapted to Slideaxially on and off said pins, a hollow metal plug xed on said stationarypart, a tube xed to said plug and extending axially therefrom, a hollowhousing fixed to the end of said tube remote from said plug, said hollowhousing having an opening in its under side, a plug secured in thisopening, said plug being formed as a socket on its inner side, a ratchetwheel within said housing, said ratchet wheel having a cylindrical stemengaging the interior of said socket, fusible metal in the base of saidsocket engaging the end only of said stem, a washer located in said tubeadjacent the end of said housing, a second washer located in saidstationary part, a push rod in said tube extending through both washersand engaging said movable part, said push rod having a hooked portionfor engaging a tooth of said ratchet wheel, and a pair of shouldersspaced apart along the length of the rod, one of said shoulders engagingsaid second washer when the fusible metal melts, and a spring engagingbetween the other of said shoulders and the washer located in said tubeadjacent the housing.

11. An electrical thermal switch comprising a plug and socket connectorincorporating a stationary part having a pair of contact pins, thestationary part having an axial opening disposed midway between saidpins, a movable part having a pair of sockets adapted to slide axiallyon and off said pins, a hollow metal plug fixed on said stationary part,a tube fixed to said plug and extending axially therefrom, a hollowhousing xed to the end of said tube remote from said plug, said hollowhousing having a socket disposed at its lower side, a ratchet wheelwithin said housing, said ratchet wheel having a cylindrical stemengaging the interior of said socket, fusible metal in the base of saidsocket engaging the end only of said stem, an abutment in said tube, apush rod extending through said tube into said housing and having oneend engaging said movable part, said push rod having a shoulder and ahooked portion for engaging a tooth of said ratchet wheel, and a springon said push rod acting between said shoulder and said abutment.

12. An electrical thermal switch comprising a plug and socket connectorincorporating a stationary part having a pair of contact pins, thestationary part having an axial opening disposed midway between saidpins and a movable part having a pair of sockets adapted to slideaxially on and off said pins, a hollow metal plug fixed on saidVstationary part, a tube xed to said plug and extending axiallytherefrom, a hollow housing xed to the end of said tube remote from saidplug, a metal tube of loop form having its ends secured to said hollowmetal plug, said metal tube containing a resistance winding the ends ofwhich are secured to said contact pins, said hollow housing beingsecured to said loop form metal tube, said hollow housing having asocket disposed at its lower side, a ratchet wheel within said housing,said ratchet wheel having a cylindrical stem engaging the interior ofsaid socket, fusible metal in the base of said socket engaging the endonly of said stem, an abutment in said tube, a push rod extendingthrough said tube into said housing and having one end engaging saidmovable part, said push rod having a shoulder and a hooked portion forengaging a tooth of said ratchet wheel, and a spring on said push rodacting between said shoulder and said abutment.

13. In a safety appliance for electrically heated apparatus, a baseadapted to support a heating element, an insulating member carried bysaid base, a plunger bar guidingly supported in said member forreciprocable movement and having a tooth at its innermost end, a springfor biasing the movement of said plunger in an outward direction, atoothed ratchet wheel carried on a shaft, a tooth of said wheel beingengageable by the tooth of said plunger when the plunger is depressedagainst its spring, a compartment in the region of said shaft, meansincluding fusible material in said compartment for normally preventingrotation of the ratchet wheel when the material is in solid form butpermitting rotation of the wheel when the material is in liquidcondition at abnormal temperatures, and a manually operable externallypositioned circuit controller arranged in connecting position to depressthe plunger and to be moved to disconnecting position when the plungeris released relative to the ratchet wheel.

14. In a safety appliance for electrically heated apparatus, a baseadapted to support a heating member, a two-part coupling forming anelectric switch for said heating member and carried by said base, saidcoupling comprising stationary and movable elements which when movedwith respect to one another serve to open the switch and to deenergizesaid member, a push rod slidably supported in the stationary couplingelement and contacting the movable coupling element, said rod having atooth at its innermost end, means for biasing the movement of said rodin an outward direction to push the movable coupling element from thestationary coupling element, means for restraining the movement betweenthe coupling elements, said means comprising a toothed ratchet wheelcarried on a shaft and having one of its teeth engageable with the toothof the push rod when the rod is depressed against its biasing means,means including fusible material cooperating with said shaft forpreventing the rotation of the ratchet wheel under normal temperaturesbut permitting rotation of the wheel when the temperature is abnormaland the material is in melted condition whereby when the wheel isrotated the tooth of the push rod is disengaged from the wheel and therod is forced outwardly by said biasing means against the movablecoupling element to break the circuit of the heating member.

WALTER H. BULPI'IT.

